During the assembly of semiconductor packages, semiconductor chips are often attached onto carriers, such as substrates or leadframes, for processing multiple semiconductor chips at the same time. After or during attachment, electrical connections are made between electrical pads on the chips to corresponding contacts or connection pads on the substrates or leadframes. This can be done by wire bonding, or the electrical pads can be directly attached onto the contacts on the substrates or leadframes. Thereafter, it is usually necessary to protect the chips and the electrical connections from the environment by encapsulating them in a molding compound, such as epoxy molding compound (“EMC”).
In a typical transfer molding process, a substrate with the chips attached is placed into a molding system comprising top and bottom molding halves and sometimes, a middle plate. Multiple molding cavities are formed in one or both of the mold halves corresponding to molding positions where the chips to be encapsulated are located. Molding compound is introduced via mold supply pots in the molding system, and the mold supply pots are linked to the molding cavities through a system of runners and gates through which the molding compound is channeled before entering the molding cavities. A plunger is insertable into each mold supply pot and the molding compound is distributed from the mold supply pot upon compression by the plunger. After the cavities have been filled, the molding compound is allowed to set and harden.
A typical molding system 10′ has multiple runner networks 12′ with individual runners 16′ symmetrically arranged on opposite sides of each mold supply pot 14′ as shown in FIG. 1. For efficiency, at least two runners 16′ are connected to each mold supply pot 14′ on both sides of the mold supply pot 14. The numbers and cross-sectional areas of all the runners 16′ linked to each mold supply pot 14′ are the same, and there are typically an even number of runners 16′ on opposite sides of the mold supply pot 14′ which connect the mold supply pot 14′ to molding cavities 15′. Such uniformity of the runners 16′ permit a constant mold packing pressure in the liquid molding compound during transfer molding throughout each runner 16′. However, there is a constraint in spatial distribution of the molding system 10′ which limits the types of substrates that may be molded. For example, where a pair of runners 16′ is located on each side of the mold supply pot 14′, only substrates that have an even number of columns of molding positions can be molded. The difficulty is accentuated when the number of columns is a prime number which is not divisible by any other integer except the integer itself. It is desirable to devise a runner system which can incorporate odd numbers or prime numbers of runners on opposite sides of the mold supply pot 14′ to improve molding versatility.